1. Field of the invention
The present invention relates to a remote subscriber control system in a digital line switching system.
2. Description of related art
Recently, due to optical transmission technology, remote subscriber terminals located far away from a switching system can be accommodated by the switching system. Conventionally, there have been proposed two types of systems in which remote subscriber terminals located far from a switching system are accommodated thereby.
In the first type of system, remote subscriber terminals are accommodated by a switching system without interposition of software in a base station, as shown in FIG. 1. In FIG. 1, remote subscriber terminals 43 are connected by a transmitter/receiver (RT) 30 and a transmitter/receiver 20 of the remote switching system (CT) to a base station 10. The base station 10 includes a speech path network switch (NW) 1, a line concentrator (LC) 2 and a processor (CP) 3. The line concentrator 2 is connected to the switch 1 and the processor 3 controls both the switch 1 and the line concentrator 2. The switch side transmitter/receiver (CT) 20 is connected to the base station 10. The switch side transmitter/receiver 20 includes a switch side interface (IF) 21, a multiplexer/separator (MUX) 22 and an optical transmitter/receiver (O/E) 23. The multiplexer/separator 22 is connected to the line concentrator 2 of the base station 10. The remote subscriber side transmitter/receiver (RT) 30 is connected to the switch side transmitter/receiver 20 by an optical transmission/receiver 30 includes an optical transmitter/receiver (O/E) 31 and a multiplexer/separator 32. Subscriber line interface circuits (SLC) 42 are coupled to the multiplexer/separator 32 of the remote subscriber side transmitter/receiver 30 via a subscriber line interface (SLCIF) circuit 41. Each of the subscriber line interface circuits 42 is connected to one of subscriber terminals 43.
In this system as shown in FIG. 1, the line concentrator 2 of the base station 10 is coupled to the subscriber line interface circuits 42 via the optical transmission path 40. The time division multiplexing of subscriber speech signals and subscriber control signals is performed, so that the subscriber speech signals and the subscriber control signals are respectively transmitted in speech time slots and in control time slots via the optical transmission path 40. In a case where speech signals (V0, V1, . . . , and Vn), control signals (SD/SCNO, SD/SCNI, . . . , and SD/SCNn), a frame pulse and a clock signal as respectively shown in (a), (b), (c) and (d) of FIG. 2 are supplied from the base station 10 to the switch side transmitter/receiver 20, the multiplexer/separator 22 multiplexes the speech signals (V0, V1, . . . , and Vn) and the control signals (SD/SCNO, SD/SCN1, . . . , and SD/SCNn) in a format as shown in FIG. 3. The multiplexed signals are output from the optical transmitter/receiver 23 to the optical transmission path 40. In the remote subscriber side transmitter/receiver 30 receiving the multiplexed signals, the optical transmitter/receiver 31 inverts the received signals into electric signals, and separates the speech signals and the control signals from each other. The separated speech signals and the control signals are supplied to the subscriber line interface circuit 41. The subscriber line interface circuit 41 successively supplies the speech signals and the control signals to each of the corresponding subscriber line interface circuits 42. Each of the subscriber line interface circuits 42 supplies the speech signal and the control signal to a corresponding one of the subscriber terminals 43.
In the second type of system, as shown in FIG. 4, the line concentrator (LC) is provided at the remote subscriber side, and the line concentrator (LC) is coupled to the switch via either an optical transmission path or a PCM transmission path. Referring to FIG. 4, a base station 50 includes a network switch 1, processor 3 and a remote controller 4. The processor 3 controls both the network switch 1 and the remote controller 4. The switch side transmitter/receiver (CT) 20 coupled to the switching system 50 is provided with the interface 21, multiplexer 22 and optical transmitter/receiver 23. A remote subscriber side transmitter (RT) 60 is connected to the switch side transmitter/receiver 20 by the optical transmission path 40. The remote subscriber side transmitter 60 is provided with the optical transmitter/receiver 31, the multiplexer/separator 32, a line concentrator 33 and a processor 34. The processor 34 controls the line concentrator 33 in accordance with instructions supplied from the base station 50. The line concentrator 33 is coupled to the subscriber line interface circuits 42 via the subscriber line interface circuit 41. Each of the subscriber line interface circuits 42 is connected to a corresponding one of the subscriber terminals 43 by a cable.
In this system as shown in FIG. 4, the line concentrator 33 is provided at the remote subscriber side, and the line concentrator 33 is coupled to the switch 1 via either the optical transmission path 40 or the PCM transmission path. The local connection of the remote subscriber terminals 43 is controlled by the processor 34 provided in the remote subscriber side transmitter 60, the processor 3 of the base station 50 controls the remote subscriber terminals 43 in response to transmit calls.
In the former, first type of system, as the speech signals are multiplexed in accordance to a predetermined format, and are transmitted between the line interface subscriber circuits 42 and the line concentrator 2, the remote subscriber terminals can be accommodated by the switch 1 without the interposition of softwares in the base station. However, as both the subscriber speech signals and the control signals must be transmitted via the optical transmission path under a condition in which the subscriber speech signals and the control signals correspond to each other, a transmission format of signals in the optical transmission path 40 is restricted. Further, time slots for the control signals and the speech signals in the transmission format are fixed. Thus, even if remote subscriber terminals are in a no calling state, control signals must be transmitted, in the time slots for the control signals, to the remote subscriber terminals in the no calling state. Thus, the optical transmission path 40 is not effectively used for data transmission.
In the later, second type of system, as the line concentrator 33 and the processor 34 for controlling the line concentrator 33 are provided at the remote subscriber side, the distribution of speech signals to the remote subscriber terminals can be controlled by the processor 34. Thus, speech signals can be transmitted in arbitrary time slots, so that the time slots can be effectively used for transmitting the speech signals in the optical transmission path 40. However, software used by the processor 34 is needed in order to control the distribution of speech signals to the remote subscriber terminals. The software must be used in cooperation with software used in the base station 50. Thus, in a case where new functions for communication are added to the system, the software must be updated. In addition, control information must be transmitted from the base station 50 to the processor 34 in order to use the software. That is, a control path through which control information is transmitted must be provided in the optical transmission path 40. As a result, the subscriber call traffic is limited by the performance of the control path.